U. S. Pat. No. 5,477,250 issued on Dec. 19, 1995 discloses a printing device. The printing device includes a rotatable cylinder or toner bearing member retaining charged toner particles or printing particles on its outer periphery, and a backing electrode spaced apart from the toner bearing member. The backing electrode is electrically connected to a power source, thereby forming an electric field for attracting the charged toner particles born on the toner bearing member toward the backing electrode. Interposed between the toner bearing member and the backing electrode is an insulating plate having a plurality of apertures through which the toner particles can pass. The insulating plate is provided with individual electrodes each of which electrodes surrounding the corresponding aperture.
In operation of the printing device, when a voltage is applied to the individual electrode in response to a printing signal, the toner particles on a portion of the surface of the toner bearing member facing the electrodes are energized. The energized toner particles are propelled from the toner bearing member into the corresponding apertures, and then passed through the corresponding apertures. Subsequently, the propelled toner particles are deposited on a recording sheet travelling through a passage between the substrate and the backing electrode, thereby forming an image corresponding to the image signal on the recording sheet.
The device, however, has a drawback that the toner particles diverge while being propelled from the toner bearing member and, as a result, each resultant dot formed on the recording sheet tends to be unnecessarily large in its size and low in its density. Further, according to the printing device, an amount of the toner particles to be propelled from the toner bearing member and a size of the resultant dot formed on the recording sheet can be controlled simply by controlling a duration for applying a voltage to the individual electrodes in response to the printing signals. However, this is far from useful for solving the above-mentioned drawback of the divergence of the toner particles.